Brake Malfunction

[JAXTURBO]

Perhaps my search skills are fading because I couldn't find what I needed here and I know the answer is here somewhere. I'm experiencing what feels like brake fade. No matter how hard I press, there seems to be no resultant increase in braking force. Master cyl is about 20 years old. Pads and rotors are brand new hawk/Zimmerman set up. Everything else is circa '86.
Found some where here that if the pedal doesn't move at all during start up, then the booster is perhaps faulty.
Should I replace the $22 check valve for starters?
Also noticed this sweating...

[mikepellegrini]

It could also be a vacuum problem.

I went through all that on my 951. I just wasn't getting any assist, so I figured the booster was bad. I went through the whole trip of replacing the booster - which is kinda a pain in the ass. I got the new booster in and there still wasn't any assist.

Upon further investigation, I found the venturi had been leaking, so I did a delete (rerouted the vacuum lines to do away with the venturis) so the vacuum for the booster was coming direct off the middle outlet of the intake.

Then the brakes worked perfectly.

I'm not sure why they used the venturis - The ***** is that when the venturis get old, they leak.

So you might wanna check your vacuum lines.

[JAXTURBO]

Originally Posted by mikepellegrini

It could also be a vacuum problem.

I went through all that on my 951. I just wasn't getting any assist, so I figured the booster was bad. I went through the whole trip of replacing the booster - which is kinda a pain in the ass. I got the new booster in and there still wasn't any assist.

Upon further investigation, I found the venturi had been leaking, so I did a delete (rerouted the vacuum lines to do away with the venturis) so the vacuum for the booster was coming direct off the middle outlet of the intake.

Then the brakes worked perfectly.

I'm not sure why they used the venturis - The ***** is that when the venturis get old, they leak.

So you might wanna check your vacuum lines.

Thanks Mike! You have any pix of what your rerouted vac lines look like? Probably a much easier job with the intake off right?

[djnolan]

If you look carefully the venturi is a two piece design with an o-ring where the pieces join. New one is not very much. Don't know about rebuilding it.

[mikepellegrini]

Here you go: VACUUM DIAGRAMS at LINDSEY RACING - Your Porsche Performance Parts Center

Yeah, it's necessary to take the intake off to do the venturi delete.

To check to see if that's it, you could simply take off the current brake booster hose, plug it, then run a new hose direct to the middle vacuum port on the manifold. You'd also have to plug the end of the existing hose (so there are no vacuum leaks).

Then with the vacuum jumper in place, see if it makes a difference. That'd tell you if it was worth it to pull the manifold.

Mine it was like night and day.

[JAXTURBO]

Originally Posted by mikepellegrini

Here you go: VACUUM DIAGRAMS at LINDSEY RACING - Your Porsche Performance Parts Center

Yeah, it's necessary to take the intake off to do the venturi delete.

To check to see if that's it, you could simply take off the current brake booster hose, plug it, then run a new hose direct to the middle vacuum port on the manifold. You'd also have to plug the end of the existing hose (so there are no vacuum leaks).

Then with the vacuum jumper in place, see if it makes a difference. That'd tell you if it was worth it to pull the manifold.

Mine it was like night and day.

Thanks. Much appreciated.

[JAXTURBO]

Alright... did the venturi delete thing. If for nothing else, things got a bit tidier and much better looking under the hood - me likes. But alas it did not address my problem. The brake pedal does drop a bit on start up. So I am assuming that I am getting some brake assist. Any tips on checking brake master and booster?

[Rasta Monsta]

Can you smoke test the booster? Did you check the function of the check valve?

[JAXTURBO]

Originally Posted by Rasta Monsta

Can you smoke test the booster? Did you check the function of the check valve?

Appreciate the assist... I actually bought a replacement valve (see below). Problem is that there is a hard line running right in front of the valve. All I can say is that when I moved the valve in an attempt to remove it, I did hear a hissing sound. Assumed that means the booster was fully vacuumed and holding same.

[marko]

I was just curious, did you try the start up test that you mention in your first post?
Pump the brake pedal, hold it, then start the car. If the brake pedal doesn't move then there is a booster problem (or vacuum problem). If it does move down some, then the booster is probably ok.

[JAXTURBO]

Originally Posted by marko

I was just curious, did you try the start up test that you mention in your first post?
Pump the brake pedal, hold it, then start the car. If the brake pedal doesn't move then there is a booster problem (or vacuum problem). If it does move down some, then the booster is probably ok.

Yup I did. I didn't pump though. I just held the pedal down while starting. Upon starting, the brake pedal did drop a bit. So I assume the booster is doing its job. The brake master was changed once due to a leak. But that was well over 20 years ago. I've never rebuild the calipers. I have swapped brake fluid either 2 or 3 times since "birth".

[JAXTURBO]

Brake fluid looked real nasty. It was very dark. I mean dark. So I flushed/bled. Noticed a creaking sound coming from the left front. Wondering if I need to rebuild that caliper? Interesting thing is that the car doesn't pull to one side or the other, so I don't think that creaking sound was a hanging caliper piston. Rebuild?